Position encoder using statistically biased pseudorandom sequence

ABSTRACT

A position sensor encodes absolute position via n consecutive members of a pseudorandom sequence of bits, where each bit (30a-30k) comprises a region of high transmission or reflectivity adjacent to a region of low transmission or reflectivity. The pseudorandom sequence is chosen such that every series of n consecutive bits (30a-30k) in the sequence is predominantly formed from a predetermined bit value (1 or 0). The light transmitted or reflected from the series is therefore detected as a substantially periodic intensity pattern (42), which can be processed via Fourier analysis to yield an accurate interpolation of relative position. An example is given of a pseudorandom sequence, which can be represented as a cyclic Manchester code, in which at least 8 members of every series of 11 consecutive bits has a bit value of 1.

FIELD OF INVENTION

This invention relates to a position sensor for the sensing of theabsolute position of a movable body without requiring counting from areference mark. In particular the invention relates to a barcode systemwhere a linear array of detectors is used to read and process theabsolute position of a surface to which the barcode is attached to highaccuracy. The barcode is coded with values that are used to find theabsolute position of the barcode marking within the length of thebarcode, and has the ability to be interpolated to provide a resolutionof measurement better than that of the barcode marking (or spacing)pitch, or indeed better than that of the individual detector pitch ofthe linear array of detectors.

BACKGROUND

Conventionally, position sensors have been used for sensing the positionof a body that is movable relative to the sensor. These sensorstypically consist of a detector unit and a graduated scale of materialwith contrasting bars formed of alternating transparent and opaque bars,or bars of alternating high and low reflectivity, the displacement ofwhich is detected by the detector unit.

The scale is typically illuminated by a source of electromagneticradiation (EMR), typically UV, visible or IR light, that in turngenerates an image in in the form of a pattern on one or more arrays ofphotodetectors sensitive to the EMR. Such arrays include CCD devices,VLSI vision chips, one and two dimensional photodetector arrays andlateral effect photodiodes (commonly referred to as PSD's or positionsensitive devices). The output of the one or more arrays is processed toproduce a measure of the position of the movable body.

Such sensors commonly provide a signal based on the incremental positionof the scale, and absolute position is determined by counting from aknown reference position. The accuracy of incremental sensors is oftensubstantially improved by the use of well known techniques such asquadrature interpolation. Such techniques generally require anon-varying bar pitch.

Alternatively, the sensor may provide a signal based on absoluteposition by the use of barcodes applied to the scale. These barcodesgenerally do not have a constant bar pitch as each set of barcodes areunique for each position to be sensed. Such absolute position sensorsgenerally do not provide the position measurement accuracy provided byincremental sensors as they cannot use the aforementioned quadratureinterpolation techniques.

If an absolute position sensor is required to have high accuracy, twoseparate scales and arrays of detectors are generally required. Thefirst measures coarse absolute position by interrogation of a barcode,and the second provides a fine relative position by quadratureinterpolation of a constant bar pitch pattern.

On the other hand International Patent Application No. PCT/AU99/00590discloses the use of a single scale for the measurement of coarseabsolute position and also for the fine relative position by quadratureinterpolation of a regular bar pattern. The angle encoder sensordisclosed in this specification is composed of a pseudo-random bar codescale of constant bar pitch and a varying bar width or, alternatively,special forms of bar codes with varying bar pitch for coarse absolutepositioning. This arrangement however has a number of disadvantages. Theuse of a barcode consisting of varying bar widths for coarse absoluteposition measurement is known to have difficulties in image processingdue to the width interpretation of the barcodes due to imperfections ofthe individual pixels that make up the detectors in the array, and thevariations in the quality of the barcode markings. Also, as the sensorrequires a finite time to integrate an image, movement of the barcoderelative to the sensor during this time period produces smearing of theimage, further degrading the quality of the image to be processed. Afurther problem is the variation of the image signal produced on thearray. This can be due to non-homogeneous illumination, non-homogenoussurface properties, or due to the combination of the positioning of allthe components that transmit, reflect or repropogate, and collect theEMR. All these effect the accuracy to which the image processing canidentify the width of the individual bars of a barcode.

For the barcode of constant width but varying pitch, the imageprocessing requires an image threshold level to be used. The setting ofa single image threshold level for the entire array for the processingof the barcode has several disadvantages. The evaluation of the binarystates of all of the bits incident on the array by the use of a singleimage threshold level is susceptible to error because the image levelson the array are due, not only to the barcode pattern, but also theeffects of non-homogeneous illumination, non-homogenous surfaceproperties, and the combination of the positioning of all the componentsthat transmit, reflect or repropogate, and collect the EMR.

SUMMARY OF INVENTION

The present invention consists of a position sensor comprising a body atleast partially surrounded by a housing, the body having a gratingelement attached thereto or integral therewith, the grating elementcomprising a surface, the surface comprising a coded distribution ofregions of high and low reflectivity or transmissibility, the sensoralso comprising at least one EMR source and at least one array of EMRsensitive detectors, the source irradiating the surface and the arrayreceiving incident EMR reflected from or transmitted through thesurface, the source and the array fixed with respect to the housing, apattern thereby produced by incident EMR on the array resulting from theregions of high and low reflectivity or transmissibility on the surfaceof the grating element, the coded distribution having a sequence ofbits, each bit comprising only one region of high reflectivity ortransmissibility and only one region of low reflectivity ortransmissibility, each bit representing a binary state 1 or a binarystate 0 depending on whether it respectively corresponds to a transitionfrom regions of high to low, or alternatively a transition from regionsof low to high, reflectivity or transmissibility on the surface of thegrating element, the bits being arranged at a substantially uniform codepitch, the incident EMR on the array therefore determining asubstantially spatially periodic intensity pattern of incident EMR onthe array, characterised in that the coded distribution comprises over50% of dominant bits of either a binary state 1 or 0, and less than 50%of regressive bits, the dominant and regressive bits arrangedconsecutively as a pseudo-random binary code, the pseudo-random binarycode sampled as a series of n consecutive bits and also determining thesubstantially spatially periodic intensity pattern incident on thearray, the coded distribution arranged such that any said series of nconsecutive bits also comprises over 50% dominant bits and less than 50%regressive bits, the pattern on the array processed by a processor toderive the absolute position of the coded distribution with respect tothe housing, and hence provide a measure of the absolute position of thebody with respect to the housing.

Preferably the substantially spatially periodic intensity patternincident on the array is interpolated by Fourier analysis by theprocessor.

Preferably each sample of n consecutive bits is unique over the range ofabsolute position of the body with respect to the housing.

Preferably the transition from regions of high to low, or alternativelya transition from regions of low to high, reflectivity ortransmissibility on the surface of the grating element is arranged inthe form of a Manchester code.

Preferably the substantially spatially periodic intensity patternincident on the array is processed to remove at least one regressive bitbefore being interpolated by Fourier analysis by the processor tofurther increase the accuracy of measurement of absolute position of thebody with respect to the housing.

Preferably the sensitivity of the EMR sensitive detectors within the atleast one array is varied according to a spatial weighting function.

Preferably the spatial weighting function is in the form of a Hanningwindow.

In one embodiment said body is rotatable about an axis fixed relative tosaid housing, and said absolute position being derived is an angularposition.

In another embodiment said body is linearly movable relative to saidhousing, and absolute position being derived is a linear position.

Preferably the pitch of the array of EMR sensitive detectors is arrangedto be smaller than the code pitch.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example withreference to the accompanying drawings, in which:

FIG. 1 is a schematic depicting the conversion of a regular 50% dutycycle coding pattern to a Manchester code;

FIG. 2 is a sequence of 15 bits, of which 12 bits are dominant bits ofbinary state 1, and of which 3 bits are regressive bits of binary state0;

FIG. 3a is a diagrammatic sectional view of an angular position sensoraccording to a first embodiment of the present invention showing arotatable body consisting of a cylindrically arranged grating elementsurface with regions of high and low reflectivity and a radiallydisposed photodetector array;

FIG. 3b is a larger scale view of a portion of the grating elementsurface shown in FIG. 3a;

FIG. 4 is a sequence of 172 bits, of which 132 bits are dominant bits ofbinary state 1, and of which 40 bits are regressive bits of binary state0;

FIG. 5 is a diagram illustrating the pattern produced by incident EMR onthe photodetector array and a technique employed in the position sensoraccording to the present invention to provide both coarse resolutionabsolute angle measurement and fine resolution interpolated measurement;

FIG. 6a is a diagrammatic sectional view of an angular position sensoraccording to a second embodiment of the present invention showing a discshaped grating element surface with an axially disposed photodetectorarray;

FIG. 6b is a larger scale view of a portion of the grating elementsurface shown in FIG. 6a.

MODE OF CARRYING OUT INVENTION

FIG. 1 depicts how a 50% regular duty cycle coding pattern containingbars 1 a-f can be modified to a Manchester code by moving one bar 1 c inthe pattern. The Manchester code can be disposed upon a surface suchthat bars 1 are of constant thickness and extend in a directionperpendicular to the intended direction of positional measurement of thebarcode. The coding pattern is shown as viewed by a known array ofdetectors (not shown). Each bar 1 and its adjacent space 2 define asingle bit 3.

The Manchester code is formed by a consecutive sequence of bits 3. Ineach bit 3, bar 1 comprises of a region of low reflectivity ortransmissibility and adjacent space 2 is of a region of highreflectivity or transmissibility. Over the whole of the Manchester codecomprising 6 bits, one regressive bit 3 b is of binary state (0). Theremaining five dominant bits 3 a are of binary state (1).

FIG. 2 depicts a sequence of 15 bits, of which 12 bits are dominant bitsof binary state (1), and of which 3 bits are regressive bits of binarystate (0). In this sequence of 15 bits, the number of bits of binarystate (1) is substantially larger than 50%, whilst the number of bits ofbinary state (0) is substantially lower than 50%. If a detector array(not shown), capable of viewing only 6 sequential bits at any instant,detects the first 6 bits (reading left to right) in FIG. 2, the codesequence is identical to that shown in FIG. 1.

Where the detector array views any sequence of 6 consecutive bits fromFIG. 2, the sequence is unique and not repeated anywhere over the lengthof that code corresponding to the range of absolute position of the bodyto which the barcode is attached. As in the case of the lefthand most 6sequential bits which have the number of binary states (1) larger than50%, any other 6 sequential bits also contain a number of binary states(1) larger than 50%.

FIGS. 3a & 3 b show an angular position sensor according to a firstembodiment of the present invention. Grating element 32 of rotatablebody 31 comprises a continuous cylindrical surface 20 on which there ismarked a coded distribution 99 composed of alternating regions of highand low EMR reflectivity, arranged in the form of a succession ofindividual binary barcodes. A metallised coating, or other shiny orlight coloured material or surface treatment, provides substantiallyaxially aligned regions of high reflectivity 21. A substantiallytransparent, roughened or dark coloured material or surface treatmentprovides the interspaced regions of low reflectivity 22. Rotatable body31 is enclosed in housing 5 and supported in bearings 6 and 7, and isable to rotate about axis of rotation 8. EMR source 10 and EMR sensitivephotodetector array 9 are fixed in housing 5 and arranged such that EMRsource 10 illuminates the regions of high and low reflectivity 21 and 22which re-radiates EMR to the substantially radially disposed array 9.Thus a pattern is produced on array 9, which is processed by processor11 to provide a measure of the absolute angular position of rotatablebody 31 with respect to housing 5.

FIG. 4 depicts a 172 bit sequence, of which 132 bits are dominant bitsof binary state (1) and of which 40 bits are regressive bits of binarystate (0). This particular bit sequence can be represented as aManchester code and used as the coded distribution 99 on surface 20 ofgrating element 32 of the angular position sensor shown in FIGS. 3a and3 b. Coded distribution 99, which is made up of the whole 172 bitsequence, extends a full 360° around grating element 32. The last bit ofthe 172 bit sequence is adjacent to the first bit of the 172 bitsequence, so that the coded distribution 99 is a continuous barcodesequence. The pattern produced on array 9 from imaging any 11consecutive bits in this continuous 172 bit sequence is unique, and doesnot occur anywhere else in the barcode sequence. Also, at least 8 bitsout of any consecutive 11 bits are dominant bits of binary state (1)while no more than 3 bits out of any consecutive 11 bits from thesequence are regressive bits of binary state (0).

FIG. 5 shows an example of a pattern produced by incident EMR on array 9according to the first embodiment of the present invention. Theindividual bits 30 a-k represent 11 sequential bits of the pattern 42 onarray 9. This array is adapted to provide both an absolute angularposition measurement and a fine resolution angular position measurement.The absolute angular position measurement is performed by the reading ofat least 11 bits so as to permit the identification of a unique angularposition of rotatable body 31.

Array 9 has pixels that are spaced at a pitch substantially smaller thanhalf the code pitch (ie. width of each bar) of coded distribution 99 andthe detected image is therefore oversampled and so the processing of thebit patterns 30 a-k does not depend on any one pixel. As conventionalarray pixels are known to vary in sensitivity, this allows for theposition sensor to work with non-optimal pixel quality. Also theover-sampling of the image provides higher resolution positionmeasurement.

The processing of the bit patterns is achieved by conventional Fourieranalysis interpolation using 2 weighting functions 41 a and 41 b.Because the incorporation of regressive bits 3 b reduces the signal awayfrom the optimal regular 50% duty cycle (as shown in FIG. 1), theFourier analysis interpolation accuracy will be slightly reducedaccordingly. In practise, the interpolation by electronics is found tobe of a higher accuracy than that of the mechanical parts, and so thisreduction of the positional accuracy is found to be of littlesignificance when compared to the mechanical run-out of rotatable body31 in bearings 6 and 7. Also, after the measurement is taken, theregressive bits 3 b can be excluded from pattern 42 to produce a secondpattern 43 consisting only of dominant bits 3 a. A Fourier analysisinterpolation is then performed on the pattern 43 which provides ameasurement based on a higher signal to noise ratio, increasing theinterpolation accuracy.

The end effect conditions are known to reduce the accuracy of theFourier analysis interpolation, hence spatial weighting function methodssuch as Hanning window scaling of array 9 can be used to vary thesensitivity of the EMR sensitive detectors and hence reduce the endeffects due to bits 30 a and 30 k.

By imaging and processing the information from 11 consecutive bits fromthe 172 bit sequence, and providing 128 positions/bit Fourier analysisinterpolation, the absolute position of array 9 relative to the codeddistribution 99 can be resolved to 22016 angular positions perrevolution (i.e. 0.016° resolution).

The Fourier analysis interpolation used to produce the high level ofinterpolation is as follows:

If the EMR intensity pattern is sinusoidal, then:

P(x)=sin[2n(x-d)/a]

Where

a=pitch of the pattern, and

d=displacement of the pattern

The pattern P(x) is sampled by the individual pixels of array 9. LetP_(i) denote the i-th sample. Thus the “pattern vector” of n samples canbe denoted as P=[P₁, P₂, P₃, . . . P_(n)].

The two weighting functions 41 a and 41 b are now defined, being thesine and cosine weighting vectors:

K _(1i)=sin(2ni/a) for i=1 . . . n

K _(2i)=cos(2ni/a) for i=1 . . . n

Hence phase angle α is given by:

α=arc tan [(ΣP _(i) K _(1i))/(ΣP _(i) K _(2i))] for i=1 . . . n

The resulting phase angle α is a measure of the displacement of thepattern relative to the sine and cosine weighting vectors and provides afine resolution position measurement that is of many times greaterresolution than the width of one bit of the pattern. This process alsoproduces similar results for spatially periodic or at leastsubstantially spatially periodic EMR intensity patterns.

Using this technique the coarse resolution absolute angular positionmeasurement and fine resolution incremental angular position measurementis combined to provide an absolute angular position measurementtechnique with fine resolution requiring only one detector array in thesensor and with low susceptibility to mechanical manufacturingtolerances.

FIGS. 6a & 6 b show an alternative angular position sensor according toa second embodiment of the present invention. In this embodiment,grating element 32 of rotatable body 31 comprises a flat disk surface 20on which is marked coded distribution 99, rather than a continuouscylindrical surface as depicted in the first embodiment shown in FIG. 3.In such an embodiment coded distribution 99 comprises a 172 bit sequencein a similar manner to that of the first embodiment.

In a further not shown embodiment, a linear position sensor may utilisea Manchester code comprising over 50% dominant bits of binary state (1)and less than 50% regressive bits of binary state (0) as described withreference to FIGS. 1, 2, 4 and 5.

It will be appreciated by those skilled in the art that numerousvariations and modifications may be made to the invention withoutdeparting from the spirit and scope of the invention.

What is claimed is:
 1. A position sensor comprising a body at leastpartially surrounded by a housing, the body having a grating elementattached thereto or integral therewith, the grating element comprising asurface, the surface comprising a coded distribution of regions of highand low reflectivity or transmissibility, the sensor also comprising atleast one EMR source and at least one array of EMR sensitive detectors,the source irradiating the surface and the array receiving incident EMRreflected from or transmitted through the surface, the source and thearray fixed with respect to the housing, a pattern thereby produced byincident EMR on the array resulting from the regions of high and lowreflectivity or transmissibility on the surface of the grating element,the coded distribution having a sequence of bits, each bit comprisingonly one region of high reflectivity or transmissibility and only oneregion of low reflectivity or transmissibility, each bit representing abinary state 1 or a binary state 0 depending on whether it respectivelycorresponds to a transition from regions of high to low, oralternatively a transition from regions of low to high, reflectivity ortransmissibility on the surface of the grating element, the bits beingarranged at a substantially uniform code pitch, the incident EMR on thearray therefore determining a substantially spatially periodic intensitypattern of incident EMR on the array, characterised in that the codeddistribution comprises over 50% of dominant bits of either a binarystate 1 or 0, and less than 50% of regressive bits, the dominant andregressive bits arranged consecutively as a pseudo-random binary code oflength m, the pseudo-random binary code sampled as a series of nconsecutive bits, wherein m and n are integers greater than one, and mis greater than n, and also determining the substantially spatiallyperiodic intensity pattern incident on the array, the coded distributionarranged such that any said series of n consecutive bits also comprisesover 50% dominant bits and less than 50% regressive bits, the pattern onthe array processed by a processor to derive the absolute position ofthe coded distribution with respect to the housing, and hence provide ameasure of the absolute position of the body with respect to thehousing.
 2. A position sensor as claimed in claim 1, wherein thesubstantially spatially periodic intensity pattern incident on the arrayis interpolated by Fourier analysis by the processor.
 3. A positionsensor as claimed in claim 1, wherein each sample of n consecutive bitsis unique over the range of absolute position of the body with respectto the housing.
 4. A position sensor as claimed in claim 1, wherein thetransition from regions of high to low, or alternatively a transitionfrom regions of low to high, reflectivity or transmissibility on thesurface of the grating element is arranged in the form of a Manchestercode.
 5. A position sensor as claimed in claim 1, wherein thesubstantially spatially periodic intensity pattern incident on the arrayis processed to remove at least one regressive bit before beinginterpolated by Fourier analysis by the processor to further increasethe accuracy of measurement of absolute position of the body withrespect to the housing.
 6. A position sensor as claimed in claim 1,wherein the sensitivity of the EMR sensitive detectors within the atleast one array is varied according to a spatial weighting function. 7.A position sensor as claimed in claim 6, wherein the spatial weightingfunction is in the form of a Hanning window.
 8. A position sensor asclaimed in claim 1, wherein said body is rotatable about an axis fixedrelative to said housing, and said absolute position being derived is anangular position.
 9. A position sensor as claimed in claim 1, whereinsaid body is linearly movable relative to said housing, and absoluteposition being derived is a linear position.
 10. A position sensor asclaimed in claim 1, wherein the pitch of the array of EMR sensitivedetectors is arranged to be smaller than the code pitch.